Picture display devices of such a type, having a picture screen on which a mixed picture signal can be displayed are known. By means thereof pictures can be displayed in which locally generated characters, drawing elements and similar items can be superimposed on a normal picture, for example a moving picture transmitted, for example, by a transmitter and received in a conventional manner. Such a signal can be generated by, for example, a teletext decoder in the display device.
It has been found that the picture jumps up and down very quickly at a transition in the vertical direction between the portion of the displayed picture which does not contain characters etc. and the portion which does contain characters etc. This phenomenon may be visible in a very annoying manner, especially when there is a large contrast in the region of the transition between the two portions. This is particularly the case for a horizontal straight line to be displayed. This "jitter" is caused by the fact that the character-containing picture consists of two equal fields, which are alternately displayed on the screen and which have a mutual difference in height of one line spacing. As a rule, this situation does not occur with a picture coming from a television transmitter, as the vertical resolution of a signal generated by a television camera is poorer than the vertical resolution of a signal generated by a digital character generator and as a horizontal straight line does, as a rule, not occur in the picture.
In the past, circuits for the elimination of the above-mentioned phenomenon were proposed which were provided at the character generator, for example a teletext decoder circuit, or in the portion in the television receiver in which the signal is processed prior to its application to the control electrode(s) of the display tube. This affects the brightness and the position of certain details of the picture to be displayed in the region of the transition to such an extent that the phenomenon becomes less visible. These circuits are, however, rather complicated.
It is an object of the invention to provide a picture display device with a reduced visibility of the jitter phenomenon and to this end, the picture display device according to the invention is characterized in that it further comprises a circuit for generating a signal for additionally deflecting the electron beam(s) in the vertical direction for substantially eliminating the interlace in a portion of the displayed picture in which also the digitally generated picture and/or text information signal is displayed.